Reciprocating motion of piston engines and positive displacement compressors have mechanical limitations on their maximum rotations per minute due to the stresses and wear incurred by reciprocating motion. Other rotary motion positive displacement devices that have rotors on parallel axes of rotation such as shown in U.S. Pat. No. 3,850,150 employ a plurality of interior rotors, however, the spurs of the interior rotors are not adapted to engage either end of the recesses of the outer rotor simultaneously for more than a single point of rotation. Therefore it is not possible to have a sealed displacement chamber in the recesses of the outer rotor.
The disclosure of U.S. Pat. No. 726,896 discloses a positive displacement inner and outer rotor scheme that utilizes a geometry of 2 to 1 for the outer and inner effective radii. This results in linear walled chambers that are parallel to reference radii of the outer rotor. This is possible only with a 2 to 1 aspect ratio which is discussed thoroughly in the disclosure below. As discussed below in the preferred embodiment, a multi-interior rotor scheme with an outer effective radius of the outer rotor greater than twice the value of the effective radius of the inner rotors can not use a linear shaped surface arrangement on the outer rotors and the feet of the inner rotors.
Other references do not disclose a proper chamber tangential width that is a function of the radial distance. When the chamber walls of an outer rotor are parallel such as in U.S. Pat. No. 728,157 the pistons can not possibly maintain a seal for any duration of rotation where the aspect ratio of the outer and inner rotors is greater than 2 to 1 but is 37 to 15 (37 pressure chambers to 15 inner pistons. Alternatively, interference will occur when the circular member of the spade shaped pistons radially extend into the pressure chambers. As described herein such a converging surface of the chamber widths allows for a seal for more than a single point of rotation.